e4defrag: Use libext2fs to get the correct superblock information

e4defrag: Use libext2fs to get the correct superblock information

Currently, e4defrag always does byte-swapping when it gets superblock
information, so the calculation of the best extents count is not
correct on little endian machine. This doesn't cause data corruption,
but it may confuse users by showing the wrong extent count.  To solve
this problem, we use ext2fs_open() instead of get_superblock_info()
that is the original function.

Signed-off-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>